As family, this product line can have huge differences in tension, throw, and size between the general makes.
My remarks are based on personal experience with three Seimitsu product (LS-32, LS-40, LS-58) and the general beginner's guide to joystick brands at the indispensible Slagcoin.com website. http://www.slagcoin.com/joystick/attributes_brands.html

The basic LS-series is the LS-32, the “general” of the group and oldest in-production Seimitsu control lever. It is by far the most commonly swapped-in replacement for the Sanwa JLF and the premiere LS-joystick as of this writing. Like most LS-series joysticks, it comes in two primary versions: a PCB version (LS-32-01) with a 5-prong adaptor for PCB harnesses which makes it instantly compatible with most in-production Japanese-style joysticks, and the basic PCB-less LS-32 version with similar microswitches to the PCB version (LS-32-01). The LS-32-01 with the SS Mounting Plate is the most commonly purchased variant for Japanese-style joystick cases; the LS-32 is what arcade operators primarily buy for installation/replacement in existing thin-metal cabinets. http://support.paradisearcadeshop.net/lib/exe/fetch.php?t=1380251198&w=500&h=500&tok=08d070&media=seimitsu-ls-32-01-arcade-joystick.jpg

There is also a “third” LS-32 variant, the LS-32-02. It really isn't a “third variant” as such. It's just an an LS-32-01 with a Flat “Re” Mounting Plate instead of the more common SS Mounting Plate. http://support.paradisearcadeshop.net/lib/exe/fetch.php?t=1380252623&w=500&h=500&tok=fa7e6d&media=seimitsu-ls-32-02-arcade-joystick.jpg
The SS Mounting Plate is the more commonly recommended mounting plate for the LS-32 series simply because it curves upwards towards and contacts the lower surface of a metal faceplate. The LS-32 has a shorter shaft than the JLF and if the SS Mounting Plate did not exist the -32 shaft would be shorter than regulation height for the joystick lever mounted in the joystick controller base.

The LS-40 series is perhaps the third most popular LS-series joystick. Dubbed the “Neo Geo” joystick, it was the control lever most often installed in Japanese-native SNK Neo Geo “candy cabinets.” It has a similar feel to the LS-32 but is a bit looser despite sharing the same spring and mounting plates as the LS-32. With just a bit more plastic in it, the LS-40 tends to have a shorter throw/somewhat faster reaction time than the LS-32 but lacks the LS-32's habit of popping the shaft or riding the restrictor gate closely. Likes most LS-series joysticks (excepting the LS-33 and LS-58 series), the LS-40 comes in a PCB variant (LS-40-01) suitable for home joystick installations and a PCB-less LS-40 variant generally installed in arcade machine cabinets.
http://support.paradisearcadeshop.net/lib/exe/fetch.php?media=seimitsu-ls-40-01-arcade-joystick-green.jpg

The LS-33 joystick, the smallest control lever produced by Seimitsu – perhaps the smallest Japanese arcade control lever in mass production, period (!) –, is the only Seimitsu control lever that does not come in a PCB-equipped version. (REPEAT: there is no LS-33-01 in production. You have to mix and match parts to make it yourself.) It is, however, compatible with the same basic PCB used by the LS-55 and LS-56 joysticks. This PCB is ironically named LS-33, too! Besides its eccentricities as an LS-series unit, by most accounts it is the LS-series unit most similiar in feel and performance to the Sanwa JLF. The LS-33 spring is unique to the unit. http://support.paradisearcadeshop.net/lib/exe/fetch.php?t=1380252590&w=335&h=300&tok=889c4f&media=ls33.jpg

The LS-56 is Seimitsu's second most popular control lever after the LS-32 and third most popular control lever overall after the Sanwa JLF and Seimitsu LS-32(-01). The LS-56 is a “love it or hate it” control unit. You either appreciate the unit's sensitivity and short engage or think it's overly sensitive and not to your liking. It has perhaps the stiffest spring of all the LS-series joysticks, the LS-56 spring, and is compatible with the widest range of mounting plates, too. The octagonal gate on the LS-56 is considered among the best gate restrictors made for any control lever and comes highly recommended by many gamers as add-on hardware for the unit. Given the way most joysticks are being built for home use now, it is most recommended to equip the LS-56-01 with the MS Mounting Plate for the highest degree of compatibility with joystick bases. The VF Mounting Plate may be used in place of the MS Mounting Plate for installations in non-universal joystick bases like the HRAP 3 and licensed HRAP 2/PS2 joystick like Hori's Arcana Heart 2 joystick and similar build licensed HRAP variant joysticks. http://support.paradisearcadeshop.net/lib/exe/fetch.php?t=1380252590&w=233&h=300&tok=33e3a2&media=ls56.jpg

The LS-58 is a redesigned LS-56 unit introduced by Seimitsu around 2 years ago. While components appear largely unaltered from the LS-56 series, the mounting plate screw-on positions have been altered and the LS-58 comes with a looser, stock spring. It is backwardsly compatible with the LS-56 spring and LS-56 octagonal gate, however. Both production series of the LS-58 come equipped with PCB's. The LS-58-01-CX comes equipped with transparent shaft covers matched to bubbletops in a variety of a colors. The LS-58-01-X features solid-colored shafts paired with solid-colored balltops. To date, only the LS-58-01-CX has been stocked in numbers outside of Japan and generally paired with the MS Mounting Plate (redesigned) for the LS-58-01 series. Even this model of LS-58-01 appears to be being dropped outside of Japan in favor of restocking the LS-56-01 amongst most domestic Japanese arcade parts vendors in the United States. http://support.paradisearcadeshop.net/lib/exe/fetch.php?t=1380254160&w=500&h=375&tok=416eac&media=base-rs.jpg
(NOTE: If gamers decide to buy an LS-58-01 after all, the best version to get is with the MS Mounting Plate. It's the second control unit from the left in picture. Another good alternate mounting plate would be the P-40 equipped LS-58-01 pictured furthest left in the sample picture. Either mounting choice should cover about every potential mounting situation in a Japanese-style joystick case. The LS-58-01 with the SS Mounting Plate is not the ideal choice. The LS-56/-58 has a taller shaft than the LS-32 so players used to regulation height with the LS-32 might not like the additional height the SS Mounting Plate option provide. With the MS and P-40 Mounting Plates, the LS-58 sits lower in the joystick base and is closer to regulation height.)

The reasons people generally switch to Seimitsu control levers are as numerous as the types of gamers out there. Some gamers prefer the faster recentering of the levers and more easily customized control lever tensions. Other gamers may prefer the Seimitsu restrictor gates and their greater variety of options over the Sanwa JLF. Still other gamers feel that the gated microswitches make a huge difference in becoming comfortable with Japanese-style joysticks. There does seem to be a consensus that it's preferrable to use certain Seimitsu control levers (LS-32, LS-56) over the Sanwa JLF for playing a variety of retro-genre games (side-scrolling platform games, side-scrolling shooting games, 2-D fighters) in addition to contemporary fighting games. For whatever his or her reasons for choosing to equip with an alternate control lever, as the Number Two Japanese arcade parts manufacturer the Seimitsu brand is a worthy alternative to any other company's product.